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Transition metal complexes: colour
Transition metal complexes
Objectives:• Describe common
shapes of transition metal complexes
• Explain why transition metal complexes are coloured
Outcomes:• Revise understanding of
the terms complex and ligand
• Describe the shape of some specific transition metal complexes
• Explain why solutions appear coloured
• Use a visible spectrum to deduce the colour of a complex ion in solution
CO-ORDINATION NUMBER & SHAPECO-ORDINATION NUMBER & SHAPE
the shape of a complex is governed by the number of ligands around the central ion the co-ordination number gives the number of ligands around the central ion a change of ligand can affect the co-ordination number
Co-ordination No. Shape Example(s)
6 Octahedral [Cu(H2O)6]2+
4 Tetrahedral [CuCl4]2-
Square planar Pt(NH3)2Cl2
2 Linear [Ag(NH3)2]+
Predict the shape of the following complexes
– [Co(NH3)5Cl]2+
– [Cr(H2O)4Cl2]+
– [FeCl4]-
Check your understanding• For each of the following complexes, give the charge on the central
metal ion and its coordination number and its name– [Co(NH3)6]3+
– [Cu(NH3)4(H2O)2]2+
– CuCl42-
– [Fe(CN)6]4-
– K4[Fe(CN)6]– Na2CoCl4
– [Co(NH3)6]Cl3
– [Cu(CN)2]-
• What shape is each of the complex ions likely to be?• Define each of the following terms, using the complex [Ni(CN)4]2- to
illustrate your answer:– Ligand– Coordination number– Shape
Complexes and colour
• Complex formation is often accompanied by a change in colour.
• Example: Adding ammonia to aqueous copper (II) sulphate: Light blue to deep blue colour change
• Q: How does colour arise?
Colour• A solution appears coloured because
certain wavelengths of visible light are absorbed by the solution
Absorbed colour
λ (nm) Observed colour
λ (nm)
Red 650 Green 500
Orange 600 Blue 450
Yellow 550 Dark blue 430
Green 500 Red 650
Blue 450 Yellow 600
Violet 400 Green-yellow 560
white lightblue and green not absorbed
a solution of copper(II)sulphate is blue becausered and yellow wavelengths are absorbed
COLOURED IONSCOLOURED IONS
a solution of copper(II)sulphate is blue becausered and yellow wavelengths are absorbed
COLOURED IONSCOLOURED IONS
a solution of copper(II)sulphate is blue becausered and yellow wavelengths are absorbed
COLOURED IONSCOLOURED IONS
a solution of nickel(II)sulphate is green becauseviolet, blue and red wavelengths are absorbed
Visible spectroscopy (Absorption spectroscopy)
• A spectrophotometer measures the absorption of light at different wavelengths
Blank
AnalyserM Sample
PM
Recorder
PM
Quartz mirror splits light beam in twoLight source
M = monochromator: selects wavelength
PM = photomultiplier: converts light into electric current
Analyser: Compares two electric currents. Any difference is dependent on the absorption of light by the sample
Recorder: pen traces absorption spectrum
What happens when light is absorbed?
• Electrons are excited to a higher energy level
What happens when light is absorbed?
4s
3 3p
3d
44p
4d
4f
In an isolated atom or ion, the isolated d orbitals have the same energy
IRON
1s2 2s2 2p6 3s2 3p6 4s2 3d6
INC
RE
AS
ING
EN
ER
GY
/
DIS
TA
NC
E F
RO
M
NU
CL
EU
S
What happens when light is absorbed?
3d ORBITALS3d ORBITALS
There are 5 different orbitals of the d variety
z2x2-y2
xy xz yz
SPLITTING OF 3d ORBITALSSPLITTING OF 3d ORBITALS
Placing ligands around a central ion causes the energies of the d orbitals to change Some of the d orbitals gain energy and some lose energy In an octahedral complex, two (z2 and x2-y2) go higher and three go lower In a tetrahedral complex, three (xy, xz and yz) go higher and two go lower
Degree of splitting depends on the CENTRAL ION and the LIGAND
•The energy difference between the d orbitals corresponds to the frequency of visible light.•The energy difference between the levels affects how much energy is absorbed when an electron is promoted. The amount of energy governs the colour of light absorbed.
3d 3d
OCTAHEDRAL TETRAHEDRAL
Theory
• ions with a d10 (full) or d0 (empty) configuration are colourless
(there needs to be at least 1 electron which can be excited AND there needs to be an empty d orbital which can be occupied when the electron is excited)
• ions with partially filled d-orbitals tend to be coloured
• it is caused by the ease of transition of electrons between energy levels
• energy is absorbed when an electron is promoted to a higher level
• the frequency of light is proportional to the energy difference
ions with d10 (full) Cu+,Ag+ Zn2+
or d0 (empty) Sc3+ configuration are colourlesse.g. titanium(IV) oxide TiO2 is white
colour depends on ... transition elementoxidation stateligandcoordination number
What happens when light is absorbed?
Plenary
• Describe two common shapes of transition metal complexes
• Explain why solutions are coloured in terms electrons
• What does the colour of a transition metal complex depend upon?
